1. Field of the Invention
The present invention relates to a servo system for positioning a read/write head over a destination track of a recording medium and, in particular, to a servo system for use in a hard disc drive.
2. Description of the Related Art
In a hard disk drive (HDD) of a conventional type, a head is moved to a destination track (destination cylinder) on a disk (recording medium) and the head reads and writes data from and into the destination track. An associated servo system performs a seek control operation for positioning the head over the destination track of the disk. The seek control operation of the servo system includes a speed control operation, transient control operation (stop control operation) and positioning control/operation (track following operation).
The speed control operation controls the moving velocity of the head to allow the head to follow a target speed in accordance with a distance from a current position to a destination track. The track following operation allows the head, which is moved by the speed control operation to the destination track, to be positioned at the center of the destination track. The transient control operation settles the state of the head when a shift is made from the speed control operation to the track following operation.
The servo system detects the current position (current track) of the head over the disk through the utilization of initially recorded servo data and performs a seek operation on the basis of the current position. The servo data is recorded on the dedicated servo surface of the disk. Some servo systems at a track following action, utilize embedded servo data recorded on the data surface of the disk.
Two head types are known, one being a servo head corresponding to a dedicated servo surface and the other being a plurality of data heads corresponding to a data surface. The servo head reads the servo data out of the dedicated servo surface and outputs it to a position signal generator. The position signal generator delivers a position signal, as an output, corresponding to the current position of the head. The servo head and respective data head are mounted on a carriage and moved in a radial direction of the disk by driving the carriage. The data head follows the servo head, which is subjected by the servo system to positioning control, and is moved to the destination track of the disk.
During the speed control in the servo system, a microprocessor (CPU) calculates, based on a position signal output from the position transducer, a target speed necessary for the head (data head) to move up to the destination track. A velocity detector differentiates a position signal output from the position signal generator and detects the actual speed of the movement of the head. An error amount (voltage signal) between the actual speed and the target speed is transduced to a drive current of a voice coil motor (VCM) for driving the carriage. By so doing, the carriage is so controlled as to be driven at the target speed. It is, thus, possible to move the head to the destination track.
Upon the movement of the head, by the speed control operation, to a proximity position which is one track behind the destination track, the servo system allows a shift from the speed control operation to the transient control operation. In the transient control operation, a velocity signal output from the velocity detector and position signal output from the position signal generator are added to produce a result of addition. The result is transduced to a drive current of VCM. The head is moved, by the transient control operation, steadily over the destination track. Then the servo system allows a shift from the transient control operation to the position control operation. In the track following control operation, a negative feedback operation is implemented to allow a position signal to become zero. It is, thus, possible to position the head at the center of the destination track.
In the track following control operation, upon the positioning of the head at the center of the destination track, an offset occurs due to the rotational variation of the disk, sometimes producing a positional displacement of the head. In order to eliminate such an offset, an integrating circuit is provided for integrating a position signal over a low-pass frequency region. A position signal, which is integrated by the integrator, has its phase compensated by a phase compensation circuit and is converted to a drive current. The phase compensation circuit is composed of a filter, such as a notch filter and lead lag filter, and adapted to suppress a resonance point in a mechanical system, such as a carriage and to prevent an oscillation phenomenon resulting from a phase delay.
In the track following control operation of the servo system, the integrator integrates a position signal so as to eliminate a offset resulting from the rotational variation of the disk. The integrator is composed of an operational amplifier and a series circuit of a resistor and capacitor and determines the time of an integral action in accordance with the capacitance of a capacitor. The offset amount varies in accordance with a destination track over which the head is occupied, that is, with the tracks classified broadly into an outer, intermediate and inner area, the offset amount varies from area to area of the disk. For this reason, it sometimes takes a relatively long time to perform an integral action by the integrator for offset elimination, though depending upon the destination track over which the head is occupied. At the time of the integral action, the head is not positioned at the center of the destination track, resulting in an unsteady state. After the track following control operation has been terminated, the servo system gives a seek end instruction to a host computer via a hard disk controller (HDC). At the time of track following control operation, a relatively long time is required, with a passage of a time for integral action by the integrator, until the seek end instruction is given to the host computer. This provides a cause for a reduced speed of access to the HDD.